1. Field of the Invention
Magnetic resonance technology is a known technique for producing images of the inside of the body of an examination subject. To that end, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field that is generated by a basic field magnet system in a magnetic resonance apparatus. The magnetic resonance apparatus further has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals and picks up the magnetic resonance signals that are generated. Magnetic resonance images are produced on the basis of these received signals.
2. Description of the Prior Art
For generating gradient fields, appropriate currents are set in the gradient coils of the gradient coil system. The amplitudes of the required currents are 50 A or more. The current rise and decay rates are up to several 50 kA/s. Given an existing basic magnetic field on the order of magnitude of 1 T, Lorentz forces act on these temporally varying currents in the gradient coils. The Lorentz forces lead to oscillations of the; gradient coil system. For a gradient coil system fashioned approximately hollow-cylindrically, a bending vibration of the gradient coil system is usually dominant. The oscillations proceed to the surface of the device via various propagation paths. There, these mechanical oscillations are converted into acoustic vibrations that ultimately lead to unwanted noise.
One development in the field of magnetic resonant technology involves fast pulse sequences that, among other things, shorten the measuring times. These cause high current amplitudes as well as high current rise and decay rates in the gradient coils. Without counter-measures, these high gradient coil currents cause pronounced Lorentz forces, leading to extremely loud noise. The fast pulse sequences control rapid and frequent changes of the direction of the current in the gradient coils. As a result, the dominant spectral components of the gradient pulse currents are shifted toward higher frequencies. If one of these components has the same frequency as an eigenfrequency of the gradient coil system, then the oscillation excitation of the gradient coil system is at a maximum and the noise that is caused is extremely loud. Such an excitation is more probable for fast pulse sequences than for slower ones.
The high oscillations caused by gradient coil currents in the fast pulse sequences can be countered, for example, with an increase in the stiffness of the gradient coil system. German OS 198 56 802 discloses stiffening the entire gradient coil system. To that end, a segmented cage is arranged between the gradient coils and the shielding coils, the segmented cage being embedded into a casting compound of the gradient coil system and being formed of axially continuous plastic profiles that can be expediently composed of fiber-reinforced or fabric-reinforced plastic. The plastic profiles can be reinforced at points by integrated fiber bundles or mats. Carbon fibers, glass fibers or Kevlar can be utilized as fibers, whereas the matrix of the plastic profiles can be composed of epoxy resin, polyester, vinylester or other thermal plastic materials.
Given a stiffening of the complete gradient coil system, a doubling of the stiffness merely yields an increase in the eigenfrequency by a factor of approximately 1.4. Since the gradient coil system is already currently a very rigid element, technological and economic limits exist as to the achievable increase of the stiffness of the gradient coil system as a whole.
Further, German OS 44 32 747 and German OS 198 29 296 disclose actuators, particularly containing piezoelectric elements, allocated to the gradient coil system whose deformation can be controlled such that deformations of the gradient coil system occurring during operation of the magnetic resonance apparatus are actively countered. The piezoelectric elements are correspondingly controlled by an electrical voltage applied to them.